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High-entropy alloys (HEA) is a class of materials consisting of multiple principal elements that often crystallize in simple lattices such as body-centered cubic, face-centered cubic and hexagonal close-packed structures. Many HEAsexhibit exceptional mechanical properties, e.g., impact toughness and ductility at cryogenic temperatures and high temperature creep strength. Out of the known magnetic HEAs, the Cantor alloy, consisting of equalparts of Co, Cr, Fe, Mn, and Ni, is by far the most investigated. The influenceof magnetism on the stability and mechanical properties for these alloys isintricate and very interesting for that reason. The concentration dependencein this alloy is, however, fairly unexplored and it would be beneficial if onecould avoid including critical elements such as Co. As such, the purpose of this work was to find a suitable replacement forCo in the Cantor alloy. Using density functional theory, the Co-concentrationwas investigated by replacing Co with the other constituent elements in different ways. It was discovered that the most energetically stable configurations,with magnetic and structural properties similar to the equiatomic alloy, werefound in the body-centered cubic phase when replacing Co with Fe.

Characterization of defect and impurity reactions, dissociation, and migration in semiconductors requires a detailed understanding of the rates and pathways of vibrational energy flow and of the coupling mechanisms between local modes and the phonon bath of the host material. Information on the inelastic microscopic interaction can be obtained by measuring the lifetime of local vibrational modes. This dissertation presents lifetime measurements of hydrogen and oxygen defects in semiconductors by means of time-resolved infrared pump-probe spectroscopy.;First, we measured the vibrational lifetime of H- and D-related bending modes in Si and other semiconductors. Time-resolved pump-probe and linewidth measurements reveal that the lifetime of bending modes can be explained by an energy gap law, i.e., the decay time increases exponentially with increasing decay order.;Second, we present the vibrational lifetime measurements of a selection of Si-H stretch modes in crystalline Si. The lifetimes of interstitial-type defects are found to be a few picoseconds, whereas vacancy-type defects have lifetimes up to 300 picoseconds. The strong dependence of lifetime on the defect structure suggests that pseudolocalized modes are involved in the relaxation of the hydrogen stretch modes in Si. It is found that the energy relaxation of Si-H stretch modes does not decay by lowest order, i.e., low frequency modes are involved in the decay process.;Furthermore, we performed lifetime measurements of interstitial oxygen in Si and Ge. The lifetime of 17Oi in Si is half of 16Oi and 18Oi. A calculation of the three-phonon density of states shows that 17Oi lies in the highest phonon density resulting in the shortest lifetime. The lifetime of the 16Oi mode in Ge is measured to be 10 times longer than in Si. The interaction between the local modes and the lattice vibrations is discussed according to the activity of phonon combination.;These studies elucidate the dynamics of energy dissipation and vibrational decay channels of point defects in semiconductors. They provide a better understanding of the dissociation of Si-H and Si-O bonds and the strong hydrogen and deuterium isotope effect found in H-passivated semiconductor devices. The experimental results provide an indispensable benchmark for future theoretical investigations.

This research deals with one aspect of the scientific field of hydrogen in metals. The study of hydrogen in metals is technologically important for many reasons, among them is the use of hydrogen in metals to store energy. Hydrogen related energy technology is particularly attractive because hydrogen can be extracted from water, our most abundant resource, and can be used without any pollution. Metal hydrides may one day be widely used for automotive propulsion in cars, in batteries, and many other energy conversion devices. Amorphous NiZr is particularly interesting for hydrogen storage because high concentrations of hydrogen can be sorbed and desorbed repeatedly with only small deleterious effects to the metal.;The motion of hydrogen and deuterium in amorphous nickel zirconium alloys (a-NiZr) has been studied and some properties of the local atomic structure in this amorphous metal have been deduced. This has been accomplished with nuclear magnetic resonance experiments measuring the spin lattice relaxation rate R{dollar}\sb1{dollar} as a function of temperature and resonance frequency. Hydrogen can diffuse into and then through metals by hopping among interstitial positions. In a-NiZr the interstitial hydrogen positions are the centers of tetrahedra, with metal ions at the corners of the tetrahedra. Diffusive hydrogen motion occurs via classical over the barrier jumps, with a Gaussian distribution of activation energies for diffusion. The existence of a distribution of activation energies for diffusion is evidence supporting the densely packed random sphere model for the atomic structure of amorphous metals. The low temperature R{dollar}\sb1{dollar} data is anomalous. Precipitation of Ni clusters out of the a-NiZr lattice may be occurring and causing unexpected spin lattice relaxation.

Complexation found in nature was the inspiration and motivation to study three model systems to gain understanding into the underlying parameters that govern these events. Static and dynamic light scattering was predominately used to understand the complexation in three model systems: complexation of antimicrobial polymers with biomimetic vesicles, the complexation of protein to a semi-flexible polyelectrolyte and with a flexible polyelectrolyte. Characterization of antimicrobial polymers in solution and their interactions with biomimetic vesicles were investigated in order to understand how antimicrobial polymers interacted with and killed bacteria. These studies observed that an aggregation of the vesicles correlated with antimicrobial activity. For these synthetic polymer systems, aggregation appeared to be a necessary component for antimicrobial activity,but was not indicative of activity. Inspired by complexation found in nature between DNA and RNA and proteins model polyelectrolyte-protein systems were also investigated. The focus of this section was to understand how polymer flexibility, concentration, protein concentration, and ionic strength affected the phase behavior and presence of soluble aggregates in solution. Construction of phase diagrams for both semi-flexible and flexible polyelectrolye systems dsDNA and hyaluronic acid showed different phase diagrams,yet amazingly both systems showed a spontaneous selection of size of ∼230nm away from any phase boundary and was irrespective of salt concentration, polymer concentration, persistence length or protein concentration. It was possible to gain insight into the internal packing of these two polyelectrolyte-protein complexes through static light scattering and fractal dimension analysis. Comparisons of the fractal dimension analysis of the DNA-lysozyme and HA-lysozyme was not affected by salt concentration and from analysis of the fractal dimension it was observed DNA-lysozyme aggregates, had a denser aggregate structure than the HA-lysozyme aggregate. It was also observed that away from the phase boundaries in each system the aggregate sizes and fractal dimensions were irrespective of polymer, salt, persistence length or protein concentration.

Nanoparticles have unique properties, compared with their bulk materials, and potential applications in nanotechnology. To realize this potential, controlled assemblies of nanoparticles into regular patterns on surfaces, at interfaces and in three dimensions are crucial. The assembly of nanoparticles at interfaces is a simple and effective strategy to realize hierarchical assemblies. In this thesis, the packing of nanorods at the interface between oil and water was investigated. CdSe nanorods were found to assemble at the oil-water interface and the structure was probed in-situ by small angle neutron scattering. When nanorod-covered droplets were dried on a substrate, the decrease in interfacial area produced an in-plane compression, and a range of two-dimensional packings of the nanorods across the droplet surface was observed, from a low-density random packing to a more-dense smectic ordering to a crystalline phase. Different orientation of nanorods at the liquid-liquid interface can be manipulated by controlling concentrations of nanorods in the bulk. For like-charged nanorods, such as TMV, repulsive forces predominate at the oil-water interfaces. The repulsion is strongly affected by the ionic strength of the bulk solution, but not the pH value in the range of pH = 6-8. Subsequent removal of the buffer solution can cause a cleavage of TMV nanorods aligned perpendicular to the oil/water interface. At polymer interfaces, mixtures of polystyrene-block-poly (2-vinylpyridine) with tri-n-octylphosphine oxide-(TOPO)-covered CdSe nanoparticles were chosen to test the theoretical prediction of synergistic effects between two self-organizing systems. The evolution of the depth-dependent ordering process was revealed by in-situ grazing-incidence small-angle x-ray scattering (GISAXS) during thermal annealing, indicating that the orientation of the microdomains began at the free surface and propagated in the film towards the substrate. Most significantly, this synergistic interaction also applies to different block copolymer morphologies such as lamellar. Further studies on gold nanoparticles show that by varying the ligand functionality of the nanoparticles, as well as the processing conditions, the distribution of gold nanoparticles in the microdomains of the diblock copolymer can be controlled.

Carbon nanomaterials, such as carbon nanotubes and graphene, have attracted significant interest over the past several years due to their outstanding and unusual combination of physical properties. These properties can be modified in a controllable way by chemical functionalization in order to enable specific technological applications. One example is hydrogenation, achieved by the exposure of these materials to a source of atomic hydrogen. This process has been considered for hydrogen storage purposes and for the control of the band gap of these materials for applications in carbon-based electronics. Hydrogen atoms are chemisorbed onto the surface of these materials, introducing sp3-hybridized C-C bonds in a structure originally formed by delocalized sp2 C-C bonding. This bonding transition causes severe structural and morphological changes to the graphene layers/walls. Also, it has been demonstrated that the exposure of multi-walled carbon nanotubes (MWCNTs) to a H2 plasma leads to the formation of diamond nanocrystals embedded within the nanotube walls. This thesis presents a computational analysis of the effects of hydrogen chemisorption on the structure and morphology of graphene and single-walled carbon nanotubes (SWCNTs), as well as of the different nanostructures that can be generated upon formation of inter-shell and inter-layer sp 3 C-C bonds in MWCNTs and few-layer graphene (FLG), respectively. The analysis is based on a synergistic combination of atomic-scale modeling tools, including first-principles density functional theory (DFT) calculations and classical molecular-dynamics (MD) and Monte Carlo (MC) simulations. The results demonstrate that SWCNTs and graphene swell upon hydrogenation and provide interpretations to experiments reported in the literature; this swelling depends strongly on the hydrogen surface coverage. A MC/MD-based compositional relaxation procedure generates configurations whose arrangements of H atoms are in excellent agreement with experimental observations. Detailed structural analysis of the hydrogenated surfaces is carried out, providing information which cannot be extracted easily from conventional experimental techniques. The findings of the analysis are used to explain the limitations on the maximum H storage capacity of SWCNT bundles upon their exposure to an atomic H flux. Furthermore, it is demonstrated that the structures resulting from formation of inter-shell or inter-layer C-C bonds are stable and provide seeds for the nucleation of crystalline carbon phases embedded into the shells and layers of the MWCNT and FLG structures, respectively. The key parameter that determines the type and size of the generated nanocrystals is the chiral-angle difference between adjacent layers/walls in the original structure. A novel type of carbon structure, consisting of fullerene-like caged configurations embedded within adjacent graphene layers, has been discovered for the case where the graphene layers are rotated with respect to each other; interestingly, one class of these structures retains the unique and desired electronic properties of single-layer graphene.

The purpose of this study is to approximate the local density of states(LDOS) for a metal block by solving the Schrödinger equation in an efficient way. To make the code more effective different methods were implemented, for example trying to parallelize the process and to run the code solely on a GPU (Graphic Processing Unit). The conclusion that was drawn was that running the code in parallel over the different orbitals on a multicore central processing unit (CPU) is faster and thusmore efficient than running it in sequential order. Running the calculations on a GPU was determined to be slower because of inefficient use of its bandwidth due to individual indexing in matrices and vectors. Further tests using block versions of the same algorithm on GPUs couldbe of interest because of better use of the available bandwidth. These tests were not done due to time constraints.

At the division of Material Physics at KTH an angle resolved photoemission spectrometer (ARPES) is being built. This system uses a pulsed laser to create ultraviolet light through higher harmonic generation. The laser has a high output effect which puts the optical components of the system under a large heat load. This thesis investigates the use of silicon carbide (SiC) as a possible material for use in the system. A diffraction grating is modelled and then processed by use of photolithography and plasma etching. This is then characterized by different methods, finally in its working environment. It is concluded that silicon carbide is a plausible material for use in the ARPES system. / Vid avdelningen för materialfysik vid KTH håller en laserbaserad vinkelupplöst fotoemissionsspektrometer på att byggas upp. Denna använder ultraviolett ljus för att excitera fotoelektroner som sedan detekteras genom time of flight. Ljuskällan som ger UV-fotonerna är en pulsad laser som ger en hög effekt, vilket ställer krav på att alla optiska komponenter i systemet skall klara en hög värmelast. I detta examensarbete undersöks om kiselkarbid (SiC) uppfyller dessa krav. Modellering av ett diffraktionsgitter görs, och resultatet av detta ger vägledning under processningen av gittret. Gittret tillverkas med hjälp av fotolitografi och plasmaetsning av en kiselkarbidwafer. Denna karakteriseras sedan med olika metoder och finns fungera bra för sitt ändamål.

For the high-power (HP) electronic applications the existing Si-based devices have reached the performance limits governed by the material properties. Hence the device innovation itself is unable to enhance the overall performance. GaN, a semiconductor with wide bandgap, high critical breakdown field, and high electronic saturation velocity is regarded as an alternative of Si. The material properties of GaN make it very suitable for fast-switching HP electronic devices and contribute to the fast growing of GaN technology. The state-of-the-art GaN devices operating up to 650 V have recently become commercially available. Further goal is to reach higher breakdown voltage which can be done via device engineering and material growth optimization. AlxGa1−xN is an ultrawide-bandgap (UWBG) semiconductor which is considered as a natural choice for next generation in the development of GaN-based HP electronic devices. This material attracts particular interest due to the possibility for bandgap tuning from 3.4 eV to 6 eV which allows nonlinear increase of avalanche breakdown field. Furthermore, both n- and p-type conductivity can be achieved on this material permitting variety of device design with reduced energy losses during operation. β−Ga2O3 is also a promising material for HP electronics because of its ultra-wide bandgap (4.8 eV) and a huge value of Baliga’s figure of merit (FOM) exceeding by far that of GaN. More interesting feature making this material attractive is the availability of low-cost natural substrates, and then the possibility to obtain high crystal quality of device structures. For the HP electronic devices thermal conductivity is one of the key parameters determining the device’s performance. The initial studies have shown that the thermal conductivity of AlxGa1−xN and β−Ga2O3 is quite low comparing with that of GaN. This is one of the biggest challenges slowing the development of these materials for HP device applications. Nevertheless, AlxGa1−xN- and β−Ga2O3-based field-effect transistors and Schottky-barrier diodes have been demonstrated showing performances superior to that of GaN. To optimize and maintain good performance and reliability, heat generated in the device active regions has to be effectively dissipated. Therefore the thermal conductivity of the materials in the device structures needs to be systematically studied and accurately determined. This information is critically important for the thermal management of the devices. Transient thermoreflectance (TTR) is a contactless nondestructive method for measuring of the thermal conductivity of materials. TTR, which is based on a pump-probe technique, has shown its potential in evaluation of the thermal conductivity in bulk crystals as well as in thin layers in hetero-epitaxial structures. The method requires an analysis of experimental data based on the fit of thermoreflectance transients with the solution of the one-dimensional heat transport equations by a least-square minimization of the fitting parameters. Such a procedure allows to extract not only the thermal conductivity of the constituent materials in the structures, but also the thermal boundary resistance at different hetero-interfaces. The main research results of the graduate studies presented in this licentiate thesis are summarized in three scientific papers. Paper I. In this paper thermal conductivity of β−Ga2O3 and high Al-content AlxGa1−xN thin layers was studied. For β−Ga2O3 the the effects of Sn doping and phonon-bondary scattering on the reduction of thermal conductivity were discussed. For the AlxGa1−xN we studied the effect of Al-Ga alloying which gives rise to phonon-alloy scattering. It was found that this scattering process accounts for low thermal conductivity of this material. Finally, a comparison for the thermal conductivity of the two materials was made. Paper II. In this paper the effect of layer thickness on the thermal conductivity of AlxGa1−xN layers grown by HVPE were investigated. Due to Al alloying the thermal conductivity of this material is degraded and reduced by more than one order of magnitude. On top of that we also observed further reduction of thermal conductivity when the layer thickness goes thinner. The mechanism of this phenomenon has been revealed by studying the phonon transport properties in bulk crystal and thin layer. Paper III. This study emphasizes the role of defects in GaN and AlxGa1−xN to the thermal conductivity of these materials. The dislocations, impurities, free carries, and random alloying have been separately studied and discussed. Thermal conductivity of samples containing these defects with various concentrations was measured and the results were interpreted by a theoretical model based on relaxation time approximation (RTA). / <p>Additional funding agencies: the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University, Faculty Grant SFO Mat LiU No. 2009 − 00971</p>